Field of the Invention
The field of invention relates to methods and apparatus to prevent failures of fiber-reinforced composite materials under compressive stresses caused by fluids and gases invading microfractures in those materials.
Description of the Related Art
Catastrophic failures of fiber-reinforced composite materials have proven to be a problem in the oil and gas industries. Now, such fiber-reinforced composite materials have now been incorporated into critically important structural components of aircraft. Such structural components include but are not limited to the wing and the wing junction boxes of aircraft. Any catastrophic failure of fiber-reinforced wings and/or wing junction boxes or other structural components during flight would likely result in significant loss of life and the destruction of the aircraft.
A problem with composites is that they catastrophically delaminate under certain circumstances. For example please refer to the article entitled “Offshore oil composites: Designing in cost savings” by Dr. Jerry Williams, a copy of which appears in Attachment No. 3 to U.S. Provisional Patent Application No. 61/270,709, filed on Jul. 10, 2009, an entire copy of which is incorporated herein by reference. One notable quote is as follows: “ . . . (the) failure modes are different for metals and composites: Compression failure modes for composites include delamination and shear crippling that involves microbuckling of the fibers.”
Based upon Dr. Williams' assessments, clearly compressive forces applied to composites can cause significant problems. Carbon fiber filaments are typically woven into a fabric material, which may be typically impregnated with epoxy resin. Such structures are then typically laminated and cured. On a microscopic level, and in compression, the carbon fibers can buckle. This in turn opens up what the applicant herein calls “microfractures” (or “microcracks”) in larger fabricated parts which are consequently subject to invasion by fluids and gasses.
Because of the risk of catastrophic delamination of composites under compression, our firm, Smart Drilling and Completion, Inc., decided some time ago to use titanium or aluminum interior strength elements, and to surround these materials with fiber-reinforced composite materials to make certain varieties of umbilicals. For example, please see FIGS. 1A, 1B, and 1C in the U.S. Patent Application entitled “High Power Umbilicals for Subterranean Electric Drilling Machines and Remotely Operated Vehicles”, that is Ser. No. 12/583,240, filed Aug. 17, 2009, that was published on Dec. 17, 2009 as US 2009/038656 A1, an entire copy of which is incorporated herein by reference. Perhaps our firm will also include embedded syntactic foam materials so that the fabricated umbilicals are neutrally buoyant in typical drilling muds for its intended use in a borehole.
Reference is made to the front-page article in The Seattle Times dated Jun. 25, 2009 entitled “787 delay: months, not weeks”, an entire copy of which is incorporated herein by reference. This article states in part, under the title of “Last months: test” the following: “This test produced delamination of the composite material—separation of the carbon-fiber layers, in small areas where the MHI wings join the structure box embedded in the center fuselage made by Fugi Heavy Industries (FHI) of Japan.” It should certainly be no news to those of at least ordinary skill in the art that this is a high stress area, and portions of these stresses will inevitably be compressive in nature.
Consequently, in such areas subject to compressive stresses, microfractures will allow, for example, water, water vapor, fuel, grease, fuel vapor, and vapors from burned jet fuel to enter these microfractures, that in turn, could cause a catastrophic failure of the wing and/or the wing junction box—possibly during flight. Similar catastrophic problems could arise at other locations including composite materials.
The counter-argument can be presented as follows: “but, the military flies aircraft made from these materials all the time, and there is no problem”. Yes, but, the military often keeps their planes in hangers, has many flight engineers regularly and continuously inspecting them, and suitably recoats necessary surfaces with many chemicals to protect the composites and to patch radar absorbing stealth materials. So, it may not be wise to extrapolate the “no problems in the military argument” to the exposure of wings and wing boxes in civil commercial aircraft, including those of the 787, to at least some substantial repetitive compressive forces that may also be simultaneously subject to long-term environmental contamination by ambient fluids and gases.
Reference is also made to the Jun. 24, 2009 summary article in the Daily Finance entitled “Is Boeing's 787 safe to fly”?, by Peter Cohan, the one page summary copy of which appears in Attachment No. 4 to U.S. Provisional Patent Application No. 61/270,709 filed on Jul. 10, 2009, an entire copy of which is incorporated herein by reference. This article states in part: “Composites are lighter and stronger hence able to fly more fuel efficiently. But engineers don't completely understand how aircraft made of composite materials will respond to the stresses of actual flight. This incomplete understanding is reflected in the computer models they use to design the aircraft. The reason for the fifth delay is that the actual 787 did not behave the way the model predicted.”
The complete article entitled “Is Boeing's 787 safe to fly?”, in the Daily Finance, by Peter Cohan, dated Jun. 24, 2009, an entire copy of which is incorporated herein by reference, further states: “Specifically, Boeing found that portions of the airframe—those where the top of the wings join the fuselage—experienced greater strain than computer models had predicted. Boeing could take months to fix the 787 design, run more ground tests and adjust computer models to better reflect reality.” This article continues: “And this is what raises questions about the 787's safety. If engineers continue to be surprised by the 787's response to real-world operating stresses, there is some possibility that the testing process might not catch all the potential problems with the design and construction of the aircraft.”
Significant problems have occurred in the past during the development of new airframes. For example, inadequate attention was paid the possibility of high stresses causing catastrophic metal fatigue during the development of the de Havilland Comet. High stresses were a surprise particularly around the square window corners. Such failure of adequate attention resulted in several notable crashes.
Another example is the explosive decompression in flight suffered by Aloha Airlines Flight 243. Water entering into an epoxy-aluminum bonded area caused the basic problem. Consequently, an epoxy resin failure between two laminated materials (in this case aluminum) has caused significant problems in the past.